Current conducting elements



July l1, 1967 c. E. RANsLEY 3,330,756

CURRENT CONDUCT I NG ELEMENT S Original Filed Dec. 16, 1960 3Sheets-Sheet l IN V EN TOR.

A TTORNEY Jly 11, 1967 c. E. RANsLEY 3,330,755

CURRENT CONDUCTING ELEMENTS Original Filed Deo. 16. 1960 5 Sheets-Sheet2 IN V EN TUR.

4 TTEWE Y July 11, 1967 c. E. RANsLEY CURRENT CONDUCTING ELEMENTS .'5Sheets-Sheet 5 Original Filed Deo.

mk m 0 T m N T E VM A N Ilm.

United States Patent O 3,330,756 CURRENT CONDUCTING ELEMENTS CharlesEric Ransley, Chesham Bois, England, assignor to British AluminiumCompany Limited, London, England, a company of Great BritainContinuation of application Ser. No. 76,265, Dec. 16, 1960. Thisapplication Mar. 21, 1966, Ser. No. 535,902 Claims priority, applicationGreat Britain, May 4, 1951, 10,548/51, 10,549/51; Ang. 3, 1951,18,490/51; Apr. 5, 1952, 9,474/52; Jan. 14, 1954, 1,154/54, 1,155/54;Mar. 10, 1955, '7,135/55, 7,136/55, 7,137/55; Nov. 28, 1960, 40,853/60 2Claims. (Cl. 2044-279) This application is a continuation of myapplication Ser. No. 76,265 tiled Dec. 16, 1960, and now abandoned.

This invention relates to improvements in solid current conductingelements -for use in the production of aluminum and methods ofmanufacturing such elements and is particularly concerned with suchelements intended to extend into the interior of an electrolyticreduction cell for the production of aluminum or a three-layer refiningcell, such an element constituting the cathode of a reduction cell ortaking part in the supply of electrolyzing current to a body of moltenmetal either in a reduction cell or in a refining cell. Such elements,whether used as cathodes or current supply leads, all come into contactwith molten aluminum at some part of their surface.

In the specifications of British Patents Nos. 784,695, 784,696, and802,471 there are described various such current conducting elementswhich have the common feature that they are largely composed of at leastone of the materials titanium carbide and zirconium carbide. In thespecification of British Patent No. 802,905 it is stated that such acurrent conducting element may also be made having at least part of itssurface intended to be exposed to the interior of the cell largelycomposed of at least one of .the materials in the group consisting ofthe borides of titanium, zirconium, tantalum and niobium and particularreference is made to the excellent properties of titanium diboride TiB2.In particular, it has a much lower electrical resistivity than titaniumcarbide, is more resistant to oxidation in the temperature range 300 to800 C., and has a lower solubility in molten aluminum at thetemperatures found in aluminum producing electrolytic cells7 i.e., 950to 1000 C. This last property is extremely important in the commercialexploitation of such current conducting elements since the life of theelements is ultimately determined by the rate at which the materialthereof dissolves in the aluminum in the electrolytic cell.

Although the borides of the elements referred to are significantlysuperior to the carbides thereof for the purposes in View they were,until hitherto, more expensive to produce than the carbides. In thespecication of British Patent No. 826,635 l have described and claimed acurent conducting element rfor use in an electrolytic cell for theproduction of aluminum, which element is intended to come into contactwith molten aluminum during its use, characterized in that at least thatportion of the element which is to contact the aluminum is largelycomposed of a mixture of titanium carbide and titanium boride. Such amixture has various advantages which could not be anticipated from astudy of .their properties detedmined separately and special referenceis made to the surprising reduction in the solubility of titaniumcarbide in aluminum at high temperatures (e.g., at 970 C.) which can beachieved by small percentage additions of titanium horide, additions ofto 25% by weight of titanium boride being particularly mentioned.

It was found that the oxygen content of the carbides 3,330,755 PatentedJuly 11, 1967 ICC of titanium, zirconium, tantalum, and niobium is animportant factor in the solubility of the compounds in molten aluminum.In my co-pending application Ser. No. 764,725 filed Oct. 1, 1958 nowPatent No. 3,215,615 I have described and claimed a solid currentconducting element for use in an electrolytic cell for the production orpurification of aluminum and adapted to have at least a part thereofexposed to a body of molten aluminum or aluminum alloy within the cellwherein at least said part of said element consists essentially of atleast one of the materials in the group consisting of the carbides oftitanium, zirconium, tantalum and niobium and the oxygen content of saidmaterials is less than 1% by weight.

The expressions largely composed of and consisting essentially of asused hereinafter in the specification and the claims, mean that thecurrent conducting element, or at least the portion thereof which isadapted to be in contact with molten aluminum, made of at least one ofthe carbides and/or borides referred to above does not contain othersubstances in amounts sucient to materially affect the desirablecharacteristics of the current conducting element, although othersubstances may be present in minor amounts which do not materiallyaffect such desirable characteristics, for example, small proportions ofcarbon, nitrogen, or iron. In order for the element to consistessentially or be largely composed of at least one of the carbides and/or borides as described above the oxygen impurity content should belimited as hereafter prescribed. Further, these expressions are used todenote that the current conducting element, or at least the portionthereof adapted to contact molen aluminum during use, desirably, but notessentially contains at least by weight of at least one of the carbidesand/or borides referred to.

Although the borides of the chemical elements refered to are markedlysuperior to .the carbides thereof for the purposes in view, myexperiments on the manufacture of solid current conducting elementslargely composed of titanium diboride and titanium diboride-titaniumcarbide mixtures, which are technically of great importance, have shownthat unexpected diiliculties can occur in the control of the quality ofsuch elements. Thus in my experimental work, a proportion of theelements were found to be quite unsuitable for use in reduction orrening cells because they failed prematurely under cell operatingconditions, this proportion being suihciently high to reliect adverselyon the possibility of manufactun ing such elements commercially. Thisfailure was associated with a clearly defined tendency to crack anddisintegrate within the cell. Micrographic examination of elements orbars after service showed that in poor materials exhibiting thisdeficiency, a characteristic intergranular penetration of aluminum hadoccurred which eventually led to complete disruption of the structure.This eiiect was absent in materials which behaved in a satisfactorymanner, even when such materials were sufliciently porous to allow somealuminum to infiltrate into the interior in use, because even underthese conditions the aluminum remained localized in the pores, .and didnot spread between the grains. These effects could be produced in moltenpure aluminum as Well as under electrolytic conditions. This qualityfactor could in fact be characterized in terms of the so-called dihedralangle (as dened, for example, in the paper by C. S. Smith in theTransactions of the American Institute of Mining and Metallurgy 1948,volume 175, page l5) made by the aluminum phase at its yconjunction witha grain 'boundary in the structure. When this angle was about 60 ormore, no penetration between the grains occurred and the material wasfound suitable for cell applications; when, on the other hand, thisangle was less than this (eg. 20-30) disintegration of the structureinevitably occurred and the material was quite unsuitable for cell use.

In order to indicate how good and poor materials may be. distinguishedby appropriate chemical analysis and to give an understanding of thecomplex constitution of these materials, it is desirable Vto indicatethe processes by which they may be manufactured.

Continually improving techniques have materially reduced the cost ofpreparing titanium diboride, TiB2, and various processes are availablefor its preparation. The most important ones are considered to be thecarbothermic process, electrolytic separation and direct reactionbetween the chemical elements. The carbotherrnic process may involve anyone of several reactions, one of which involves reacting TiO2 (anataseor rutile), B203 and carbon according to the following equation:

An alternative reaction is one in which the boron is supplied in theform of boron carbide (nominally B4G):

Elemental boron may also be used in accordance with the equation:

TiO2+2B|2C TiB+2CO (3) Boron, however, is diicult to produce in a pureenough form for the purposes in view, and this is not at presentregarded as an economic route.

An alternative reaction which may be employed is ments:

Tia-2B m32 (5) In this case again, however, the main objection is thedifficulty of Ipreparing elemental boron of the necessary purity at aneconomical price.

Various electrolytic processes have also been proposed for thepreparation of the borides. These do not normally offer any technical oreconomic advantage over the carbothermic or direct reaction processes;however, even for this product, the considerations to be detailed beloware still applicable.

It is extremely stoichiometric balance in diicult in practice to achievean exact a carbothermic, or any other reaction for the preparation ofTiB2, as is implicit, for example, in Equation 2 above. There aretechnical diiculties, for example, in providing exactly the correctamounts of boron and carbon; this is because somewhat variable lossescan occur during the reaction, accidental contamination may take place,and the atmosphere in which the process is carried out may also have aneffect on the composition of the nal product. A further possibility isthat the reaction may not have proceeded as far to completion as themake-up of the reaction mass would allow under appropriate conditions.

The product obtained from such reactions will thus contain, to a greateror lesser degree, components other that the simple compound TiBZ. Thus,if the reaction mixture contains an inadequate supply of boron or boroncompounds to combine with all the titanium present, and carbon ispresent over and above that required for the elimination of oxygen, theproduct will contain an appreciable amount of titanium carbide, TiC. Infact, it is probably not inaccurate to regard TiC as a primary productof the carbothermic preparation; it tends to be unstable in the presenceof boron, however, in conformity with the reaction:

Tic+2n ri2+c (6) Similarly, if the mixture is decient in both boron andcarbon, the product will be contaminated with oxygen in some form orother. The yother possible variations in composition which can occur donot need detailed description, but it will be evident that the finalproduct may possibly contain, for example, exces lboron carbide and free(or unccmbined) carbon. In addition, a number of contaminating elementsmay be present which are derived from the raw materials or from theatmospheres used; these may include, for example, relatively smallquantities of nitrogen, iron, calcium, silicon, and aluminum.

rhe final product of such carbothermic or other processes is normally inthe form of a relatively ne powder, which has been subjected to millingor other operations to render it suitable for further fabrication.

The solid conducting elements referred to are produced by either coldpressing followed by sintering, or by hot-pressing this powder, anappropriate protective atmosphere being used in both these operations.For example, it may be hot-pressed in a graphite die, in a protectiveatmosphere of hydrogen, and at temperatures of the order of 1800-2100"C. It will be understood that the chemical composition of the finalelement thus obtained will be determined not only by the exactcomposition of the powder used, but also by the conditions under whichthe element is manufactured from it. Precise control of the finalcomposition of the element is essential, however, if the latter is tohave a useful economic life in an electrolytic cell.

We have found, unexpectedly, that the elements exhibiting poor behaviordo so because of the presence therein of a relatively small percentageof oxygen combined in a certain manner, and it has further been foundthat the permissible content of oxygen of this type is related to theso-called soluble, or combined, carbon content of the material. Thediscovery of this inter-relation between the oxygen and carbon contentsof such elements has enabled us for the first time to control theircomposition in such a way that no cracking or disintegration occurs inservice.

According to the present invention, a solid conducting element for usein the electrolytic production or purification of aluminum having atleast a portion consisting essentially of titanium diboride or a mixtureof titanium diboride and titanium carbide and has an oxygen impuritycontent, defined as that oxygen present in the element in chemicalcombination with titanium which is less than 0.1% by weight when thecombined (or acid-soluble) carbon content is less than 0.4% by weight,and is less than (0.1+0.04 n)% by Weight when the combined carboncontent is 11% by weight, and the value of n is greater than 0.4.

Preferably the element contains titanium carbide in the proportion ofnot more than 40% by weight and desirably not less than 2% by weight.

From the analytical point of view, the proportion of acid-soluble carbonpresent defines the combined carbon content, which is normallyascribable to the formula TiC. In analysis, this figure is usuallyderived by earring out a determination of the total carbon content ofthe material, and a separate determination of the acidinsoluble orltrable carbon residue remaining after the mass of the sample isdissolved in an appropriate acid or acid mixture; this latter gure thusincludes graphitic and elemental carbon, and also carbon in the residualboron carbide. The acid-soluble carbon is then obtained from thedifference between these two figures, and this is the value of carboncontent which is specified above to which the tolerable oxygen contentis related.

The oxygen content of the nal element may conveniently be determined bythe well-known vacuum fusion method, in which a sample is reduced in ahigh temperature bath of molten iron or platinum contained in a graphitecrucible, and the consequent evolution of carbon oxide measured. Thesample may, for example, comprise a relatively massive chip or cutsection of the element. If the element is powdered in order to obtain arepresentative sample and for convenience in general analysis, spuriousoxygen will be introduced in the form of titanium oxide and boric oxide(B203) on the surface of the particles,` and also as absorbed water;appropriate correction must therefore be made in order to obtain theoxygen content which is significant in the present context. Methods areavailable for this; thus the B203 content can be determined by aqueousextraction of the powder, and the water content can be deduced from thehydrogen evolved in the vacuum-fusion measurement.

It is clearly essential to be able to assess the quality of elementwhich will be obtained by hot-pressing or otherwise consolidating agiven batch of powder into the said element. Since it is desired thatthe oxygen content, as dened above, of the final product shall be as lowas possible, a minimum requirement is that the proportion of reducingagents present shall be adequate to eliminate this oxygen from thepowder.

Both carbon and boron are capable of functioning as reducing agents. Forreasons which are explained in more detail below, it has been foundpreferable to adjust the composition of the charge in the carbothermicreaction so that the powder for pressing and also the nal elementcontains an appreciable percentage, e.g., 0.2% or more, of acid-solublecarbon, or about 1% or more of TiC. Oxygen will be evolved from a powderof this type during hot-pressing or sintering by reaction with the free,or other available, carbon with a loss of carbon monoxide from thesystem. Oxygen present as titanium oxide (e.g., TiO) is able to react asfollows:

and the carbon required render these conditions such that the weightratio O/C is not greater than YZF- 0.66. However, if boron is availablein the powder either as unreacted boron or boron carbide, or even asB202, further carbon is made available by reaction of this boron withTiC to form TiB2 and releasing carbon as expressed in Equation 6. Underthese conditions the basic carbon requirement is then given by thecarbon available from reaction (6) in percent by weight is 0.56 timesthe percentage of boron present in the powder other than as TiB2. Arequirement for the control of quality of the powder is thus:

Total O to be reduced Total C available for reduction Total O by Vacuumfusion Free C +0.56 X B content other than TiB2 If a mixture is used inwhich an excess of boron is intended, the TiC content of thecarbothermic powder tends to vbe low by virtue of the reaction shown inEquation 6, ie., the carbon is displaced by boron. The undesirableoxygen content can then be reduced by reaction with boron; the productobtained, B203, is not harmful to quality and since it is quite volatileat high temperatures, some is lost from the element during thehotpressing or sintering operation. There are some very inconvenientconsequences of this type of compositional control, however, whichrender it undesirable. Thus, a high B202 content tends to cause stickingof the powder in the die during hot-pressing so that a high degree ofcompaction cannot be obtained without adopting special precautions. Inaddition, it is found that the electrical resistivity of the finalelement is higher than in materials with an appreciable TiC content andthat the oxidation resistance is also poorer.

Accordingly, the present invention also provides a powder for use in themanufacture of a solid current conducting element for use in anelectrolytic cell for the production of aluminum which powder consistsessentially of at least one of the materials titanium diboride andtitanium carbide and wherein the ratio of total oxygen present in thepowder in percent by weight to the total carbon available in the powderfor reduction in percent by weight s less than 1.33.

It is preferred that the powder should contain boron in an amount lessthan that required bythe stoichiometric ratio for the titanium presentand carbon in addition to that required for reduction in an amountsucient to compensate for this deficiency of boron.

The invention also provides as a further feature, a method of producinga solid current conducting element for use in an electrolytic cell forthe production of aluminum which comprises hot-pressing or cold-pressingand subsequently sintering a powder according to either of the twoimmediately preceding paragraphs.

It is preferred to hot-press the powder to a density such that theporosity does not exceed 10% by volume although a greater porosity canbe tolerated provided it does not exceed 20% by volume. Good resultshave been achieved with a powder having a particle size which averages6/,u with a deviation of 2.5/p. but higher particle sizes can betolerated and smaller particle sizes are preferred.

As mentioned above, commercial production of titanium diboride (e.g.,using reaction (2) above) automatically introduces a number ofcontaminating elements of which the principal ones are iron, carbon,nitrogen and oxygen. Traces of other elements such as calcium, siliconand aluminum are also present, usually totalling not more than 0.5 byweight and these, in such small proportions, do not appear to besignificant. Carbon and oxygen usually derive from the basic materialand nitrogen is principally derived from air contamination andadsorption in the carbon black employed as one of the startingmaterials. Iron is derived from two main sources (a) original impurityin the starting material and (b) pick-up in grinding and ball-millingoperations. Generally speaking, the iron content should be less than0.5% by weight although a higher content can be tolerated. The nitrogencontent should be less than 0.3% by weight for a percentage by weight ofcombined carbon of up to 0.4%, not more than 0.6% by weight at 10%combined carbon (corresponding to 50% by weight of titanium carbide) andnot more than 1% by weight at 20% cornbined carbon (corresponding to byweight of titanium carbide) ,although a higher content can be tolerated.v

It is preferred that the free carbon content of the final element shouldnot exceed 0.8% by weight.`

Six examples of the manufacture of solid current conducting elementswill now be given:

EXAMPLE I (Powder M lS) Weight Equivalent Percent Weight CompoundsPercent Acidsoluble boron Titanium Soluble carbon Iron Nitrogen Totaloxygen Total In the above analysis, and in those given subsequently, theso-called acid-soluble boron includes a small proportion ofwater-soluble boron, which may be present as 7 B203 or hydrolysableborides of impurity elements, or both.

The powder was pressed in a 2 in. diameter graphite die in an atmosphereof hydrogen, with an applied pressure which was increased in appropriatesteps from 1A -ton/sq. in. to a final value of 1 ton/sq. in., thetemperature being raised from cold to 200G-205 C. in about 31/2-4 hours.

The rod obtained, which was approximately 20 in. long, had the followingproperties which could be determined by non-destructive testing:

Density (overall) (89% theoretical) 4.02 Electrical resistivity microhmcm-- 19.4

The chemical analysis of the material, carried out on a sample of therod crushed to -90 mesh B.S.S. (British standard Screen scale) was asfollows:

(Rod B. U/414) Weight Percent Equivalent Compounds Weight PercentNitrogen Total oxygen 1 As titanium oxide."

The titanium oxide content was deduced by subtracting the spuriousoxygen (associated with water vapor and surface oxidation produced bythe powdering process) from the total oxygen. A check igure obtained bymeasurement of the total oxygen content of a large fragment or chip ofbar (which required only a very small correction for spurious oxygen)gave 0.11% oxygen as ttanium oxide.

This rod was provided with an aluminum lead at one end in the mannerspecified in British Patent No. 825,443, and was used as a top-enteringlead in an experimental reduction cell as described in my U.S.application Serial No. 660,994, tiled May 23, 1957. It carried currentfor a period of 122 days without any cracking or serious deteriorationwithin the molten flux and metal zone of the cell, and with very littlesolution (a reduction in diameter of only 0.008 n.).

This was thus representative of a iinal fabricated material suitable forcell application.

EXAMPLE II (Powder 211./2) Weight Percent E qulvalent Weight CompoundsPercent The powder was hot-pressed on a similar schedule to Example I,and a 2 in. diameter rod approximately 20 in. long obtained with thefollowing properties:

Density (overall) (91% theoretical) 4.11 Electrical `resistivity microhmcm 13.6

The chemical analysis of the crushed rod was determined as follows:

(Rod B 1GO/235) Weight Percent E quivalent Compounds Weight PercentAeidsoluble boron Titanium Acid-insoluble boron Frce'y carbon Solublecarbon Iron Nitrogen Total oxygen Total 1 As titanium oxide.

The titanium oxide was deduced as in Example I. The rod was providedwith an aluminum lead as previously described, and was used as atop-entering lead in a reduction cell in the same manner as in ExampleI. It failed to carry current after a period of immersion of less than 2days, and when removed for examination showed multiple cracking, mostlyof a radial character, which rendered it useless.

Material of this nature was thus clearly quite unsuitable for cellapplications.

EXAMPLE III 2250 g. of titanium boride powder of similar physicalcharacteristics to that specied in Examples I and II were taken; thisbatch had the following analysis:

This powder was hot-pressed on a similar schedule to that described forExample I, except that because of the smaller weight of charge it waspressed in a different die assembly, and the time taken to reach themaximum temperature of about 2050 C. was only 11/2 hours.

The rod obtained Was approximately 10 in. long and had the followingproperties:

Density (overall) (89% theoretical) 4.03 Electrical resistivity microhmom-- 18.0 Transverse rupture strength tons/ sq. in 15.4

The chemical analysis of the crushed rod was ldetermined as follows:

(Rod B. 10C/297) Weight percent Equivalent Weight Compounds percentSoluble Carbon.

Iron Nitrogen Total oxygen Total to be quite sound and free from cracks.The dia-meter had decreased by 0.11 in. during this immersion. Therather higher rate of solution than that quoted in Example I arose fromthe conditions of test. In the test described in Example I, suicientTiB2 rods were inserted into the molten metal to produce saturatedsolution conditions, whereas in the immersion tests the metal was notfully saturated.

Material of this composition was thus judged suitable for cellapplications.

EXAMPLE IV 2250 g. of titanium boride powder of similar physicalcharacteristics to that speciiied in Examples I, II and III This powderwas hot-pressed on a similar schedule to that described for Example III.

The rod obtained was approximately in. long and had the followingproperties:

Density (overall) (90.5% theoretical) 4.10 Electrical resistivitymicrohm cm 14.2 Transverse rupture strength tons/sq. in 5.7

The chemical analysis of the crushed rod was deter- 10 z200 g. of thepowder were hot-pressed on a similar schedule to that described inExamples III and IV.

The rod obtained was approximately 10 in. long and had the followingproperties:

Density (overall) (95.2% theoretical) 4.36 Electrical resistivitymicrohm cm 16.6 Transverse rupture strength tons/ sq. in 14.6

The chemical analysis of the crushed rod was determined as follows:

(Rod B. 90/3) Weight Equivalent (Weight Percent Compounds Percent)Acid-soluble boron 27. 36 Titanium 68.8 Acid-insoluble boron- 0. 08 Freecarbon 0. 48 Soluble carbon 2. 37 Iron 0.15 Nitrogen. 0. 07

Total oxygen 0. 42

Total judged suitable for cell appli- EXAMPLE VI Titanium diboridepowder was ball-milled with 10% by weight of titanium carbide powder andthe iinal milled material was analyzed as follows:

uned as follows' (Powder ses) weight Equivalent weight Percent CompoundsPercent (Rod B. 10G/241) Weight Equivalent Weight percent Compoundspercent 40 Acid s01ub1e boron n en ium Agia-soluble boron 29.01 TiBn93.2 frflllffff'u jjj: Tlamum- 68- 2 Soluble carbon- {tord-insolubleboron 0-04 Iron Free Nitrogen Total oxygen- Nitrogen Teta] Oxygen 1 0.59Total oxygen 0.79

Total 99.40 2000 g. of the powder was hot-pressed on a similar scheduleto that described in Examples III, IV and V. lAs titanium oxide." Therod obtained was approximately 10 in. long and The titanium oxidecontent was deduced as in Examples I, ]I and III.

Half of this bar was subjected to an immersion test in a reduction cellin the same manner as described in Example HI. This was recovered after6 days and was found to be badly cracked and virtually disintegrated.

Material of this composition was thus clearly unsuitable for cellapplications.

EXAMPLE V Titanium diboride powder was ball-milled with 10% by weight oftitanium carbide powder, and the inal milled material was analyzed asfollows:

had the following properties:

Density (overall) (97.3% theoretical) 4.46 Electrical resistivitymicroh-m cm 14.6 Transverse rupture strength tons/sq. in..- 17.7

The chemical analysis of the crushed rod was deter- .mined as follows:

(Rod B. /1) T/Veight E quivalent Percent Weight Compounds PercentAcid-soluble boron Titanium Acid-insoluble boron Free carbon Solublecarbon on N trogen Total oiwgen Total l As "titanium oxide.

Half of this rod was subjected to an immersion test in a reduction cellin the same manner as previously described. It was recovered after 21days and was found to be badly cracked.

l l Material of this composition was thus clearly unsuitable tor cellapplications.

The oxygen content of a solid current conducting element produced asdescribed above, other than that due i2 will now be briefly describedwith reference to the accompanying drawings in which:

FIGS. 1 to 7 are sectional views of reduction cells, and FIGS. 8 and 9are sectional views of three-layer reiining to B203 or oxygen absorbedduring powdering of the elecells.

ment to analyze it, is considered to be mainly present in In the exampleillustrated in FIG. 1 pairs of opposed the form of oxygen associatedwith the metal, e.g., tisolid cathodes 1 of plate form and consistingessentially tanium oxide and it will be observed from the above exof atleast one of the materials titanium diboride and amples that theelements produced in accordance with titanium carbide are supported bywalls 2 at a small angle Examples I, III and V, all of which weresuitable for the to the vertical one on each side of an appropriatelypurposes in view, conform to the requirement that this shaped anode 3.The right-hand wall 2 is omitted in this oxygen content in percent byweight, in relation to the figure. The cathodes 1 are immersed in moltenelectrolyte combined or acid soluble carbon content of the element 4containing dissolved alumina and extend at their lower in percent byweight is less than 0.-l% when the combined end into the pool of moltenaluminum 5 which forms on carbon content is no greater than 0.4% and isless than the floor of the cell. Current leads 6 which also consist (0.l-l-0.04 n)% where nis said combined carbon conessentially of at leastone of the materials titanium ditent and is greater than 0.4%. Thisrelationship is due to boride and titanium carbide extend through thewall of the fact that when the combined carbon is greater than the cellin the pool of molten aluminum S and are con- 0.4%, the oxygen, in theform of titanium oxide TiO, nected at their outer ends to the negativepole of the D.C. can exist in solid solution in the carbide, whichappears SUPPlY- The P001 0f mOteD aluminum 5, in this eXamPle, as asecond phase. forms part of the current supply system and also acts asAn analysis of the powders used in and of the final ele. a cathode. Theusual crust of solidified llux which forms ments or rods produced by theabove examples is set out Over the Cell iS indicated at 7. in thefollowing table. In this table, total carbon available The arrangementillustrated in FIG. 2 is very similar for reduction is calculated on thebasis Total carbon=Free t0 that illuStrated in FIG- 1 and likereferences are USed t0 carbon+0.56 (acid-insol. B-i-water-sol. B).denote like parts. In this case, the current leads 6 are TABLE I PowderAnalysis, Percent by Weight Rod Analysis,

Percent by Weight Ratio Example O/C Acid Water Total C Total O asResidual Free C" insol. sol. B available O "titanium Free C Quality Bfor reduction oxide 0.70 0.23 0.15 0.91 1.11 1.2 0. 07 0.29 Good. 0.200. 07 0.47 0.50 1.33 2.05 0. 63 0.02 Bad. 0. 48 0.00 0.07 0.54 0.02 1.150.08 0. 00 Good. 0.20 0.04 0.45 0.52 1.27 2.4 0. 59 0.04 Bad. 0.84 0.440.12 1.15 1.20 1.05 0.17 0.48 Good. 0.25 0.08 0.10 0.38 0.85 2.3 0. a00. 03 Bad.

It will be observed from this table that the powders omitted andreplaced by extensions 1a of the cathodes from which a good element wasproduced conform to the which extend through the crust 7 for connectionto the requirement that the ratio of total oxygen to total carbonnegative pole of the D.C. supply. These extensions are available forreduction should be less thanv 1.33. provided with protective sleeves 8where they extend It is apparent from our investigations that in a solidthrough the crust 7, these sleeves being of aluminum or a currentconducting element consisting essentially of at refractory material.least one of the materials titanium diboride and titanium Thearrangement illustrated in FIG. 3' is similar to that carbide, thequantity of oxygen present in the form of the illustrated in FIG. 1 butin this case, the solid cathodes 1 oxide of the metal is a determiningfactor. It would ap- 50 are omitted together with their supporting walls2 and the pear that when this quantity of oxygen is higher than a poolof molten aluminum 5 constitutes the sole cathode. predetermined valuewhich is a function of the combined The arrangement illustrated in FIG.4 is similar to that or acid-soluble carbon content of the element, theele- ShOWn in lFIG. 3 but in this case the current leads 6 do ment isattacked by molten aluminum which penetrates not extend through the wallofthe cell but extend into the the element and causes it to crack and/or disintegrate. pool of molten aluminum 5 through the crust 7 at theside It will be apparent from the above disclosure that an of the celland are provided with a protective sleeve 8. It unsuitable powder can,under appropriate conditions, be is preferred, however, that the currentleads 6 should be made suitable by increasing the free carbon contenttherearranged as indicated in FIG. 5 substantially centrally of of and/or by increasing the boron content thereof (other the cell betweenanodes 3 as this arrangement leaves the than in the form of the borideof the metal). As higher sides of the cell free for replenishing thecell and the crust oxygen contents can be tolerated with increasingpropor- 7 is thinner at the center than at the sides. Additionally tionsof the carbide of the metal, a bad powder can somethis arrangement lendsitself to an arrangement of bustimes be changed to a good powder byincreasing the bars (not shown) whereby magnetic effects can be reamountof titanium carbide present. This is not a disadduced to a minimum.vantage as I have found that the very pure borides present FIG. 6 showsan arrangement in which the current diiculties in pressing to the iinalform as they do not leads 6 extend through the door of the cell. densifyreadily and there is a tendency for sticking to FIG. 7 shows how anexisting cell may be modified to occur in the graphite die. It istherefore preferred that a include current leads 6 according to theinvention, these solid current conducting element consisting of at leastone leads being embedded in the carbon floor 9 of the cell of thematerials titanium diboride and titanium carbide which is connected tothe negative pole ofthe D.C. supply should contain from 2 to 40% byweight of titanium di by iron bars 10 embedded therein. The currentleads exboride or titanium carbide and desirably not less than tend upinto the pool of molten aluminum 5 through any 10% by weight. sludgelayer which may be formed on the floor of the cell Some examples ofelectrolytic cells embodying solid and so avoid the power loss whichsuch a layer otherwise current conducting elements according to theinvention introduces.

FIG. 8 shows a three-layer refining cell having the usual lower layer 11of molten aluminum alloy, intermediate layer 12 of electrolyte and upperlayer 13 of puriiied aluminum. The lower layer 11 is connected to thepositive pole of a D.C. supply by current leads 6a extending through theWall of the cell and the upper layer 13 is connected to the negativepole of the D.C. supply by current leads 6b extending through the wallof the cell, the current leads 6a and 6b consisting essentially of atleast one of the materials titanium diboride and titanium carbide.

In a three-layer cell illustrated in FIG. 9 'the current leads 6b areshown extending into the upper layer 13 from the top of the cell and areprovided with protective sheaths 8.

It will be obvious that various modications and alterations may be madein this invention without departing from the spirit and scope thereofand it is not to be taken as limited except by the appended claims.

What is claimed is:

1. A solid current-conducting element for use in the electrolyticproduction of aluminum wherein at least a portion consists essentiallyof titanium diboride, an oxygen impurity and combined carbon contentconsisting essentially of titanium carbide, the carbon of said combinedcarbon content being present in an amount greater than 0.4% by weight ofsaid portion, the relationship of the oxygen impurity and the carbon ofsaid combined carbon content being such that the oxygen impurity contentis less than (0.1-1-0.04 n) percent by Weight wherein n is the carbon ofsaid combined carbon content.

2. A solid current conducting element according to claim 1 having atitanium carbide content of from 2%- 40% by weight.

References Cited UNITED STATES PATENTS 2,722,509 11/ 1955 Wainer 204-642,915,442 12/ 1959 Lewis 204-67 FOREIGN PATENTS 802,905 10/ 1958 GreatBritain.

JOHN H. MACK, Primary Examiner. D. R. JORDAN, Assistant Examiner.

1. A SOLID CURRENT-CONDUCTING ELEMENT FOR USE IN THE ELECTROLYTICPRODUCTION OF ALUMINUM WHEREIN AT LEAST A PORTION CONSISTS ESSENTIALLYOF TITANIUM DIBORIDE, AN OXYGEN IMPURITY AND COMBINED CARBON CONTENTCONSISTING ESSENTIALLY OF TITANIUM CARBIDE, THE CARBON OF SAID COMBINEDCARBON CONTENT BEING PRESENT IN AN AMOUNT GREATER THAN 0.4% BY WEIGHT OFSAID PORTION, THE RELATIONSHIP OF THE OXYGEN IMPURITY AND THE CARBON OFSAID COMBINED CARBON CONTENT BEING SUCH THAT THE OXYGEN IMPURITY CONTENTIS LESS THAN (0.1+0.04XN) PERCENT BY WEIGHT WHEREIN N IS THE CARBON OFSAID COMBINED CARBON CONTENT.